Plastic containers are currently more popular than containers composed of wood or inorganic materials such as metal, glass, or ceramics, for storage especially for food storage. An important factor in food storage, whether the food is prepared for storage by dehydration, freezing, or sterilization, is the complete inhibition of microbial growth. This can be achieved by sealing foodstuffs into containers with gastight seals.
Important factors critical for preserving foodstuffs in containers with gastight seals include the mechanical strength, durability, ability to maintain water and ability to minimize the effects of the atmosphere and light on the preserved foodstuff of the gastight seals (see "Ullmann's Encyclopedia of Industrial Chemistry", 25th Ed., Verlag Chemie: Weinheim, 1985, pp. 523-560 and 583-618; the applicable standards are also discussed therein).
Previously gastight seals composed a layer of aluminum coated with a sealing coating have been used to seal plastic containers holding food, particularly dairy products such as yogurt. Aluminum seals are typically comprised of a three-layered laminate. The outer layer frequently comprises biaxially oriented polyethylene terephthalate (O-PET), biaxially oriented polypropylene (O-PP), biaxially oriented polyamide (O-PA) or cellulose. The middle layer comprises aluminum. The heat-sealable inner layer adjoining the aluminum layer typically comprises polyethylene, ethylene copolymers, or polypropylene (Stehle, G. (1991) Neue Verpackung, 9:94-101). U.S. Pat. No. 4,753,708 describes heat-sealable coatings for metal foils which are suitable for sealing various substrates, such as polystyrene substrates. The coatings comprise a film-forming dispersion of a graft polymer based on an olefin and a (meth)acrylate, in an organic solvent. However, the use of aluminum for packaging has recently met with ecological and economic objections.
Accordingly, gas tight seals composed of plastic films with sealable coatings are being used. Hard polyvinylchloride (PVC) increasingly is widely used as a relatively inexpensive material for sealable films. Hard PVC has good mechanical strength and good barrier characteristics with regard to gas permeability. Customarily an acrylic resin is used as a sealing coating layer. The adhesiveness and melting point of the acrylic resin can be modified with additives.
Unfortunately the high permeability of certain plastics to gases and vapors can lead to problems in food preservation when the plastics are used as packaging materials. Multilayer films have been suggested to overcome this problem (see German Patent 35 31 036 and European Patents 0 406 681 and 0 437 745).
German Patent 35 31 036 describes plastic films produced by coextrusion comprising a sealable layer of an impact resistant polystyrene, a block copolymer, and a lubricant, possibly applied to a support layer.
European Patent 0 406 681, discusses the problems of using heat-sealable plastic films instead of aluminum laminates. As a rule, plastic seals require much narrower processing ranges (usually between 10.degree. and 20.degree. K.) than aluminum seals. The processing temperature must be continuously monitored in order to ensure problem-free production and use of the sealed package. When the containers being sealed consist of a plurality of cavities which must be simultaneously filled, such as cups or the like, processing requirements are often difficult to meet. To solve these problems, European Patent 0 406 681 describes a plastic film produced by coextrusion or roll-lamination of two or three layers (optionally separated by intermediate layers), wherein each layer contains an adhesive for binding the layers together. The film comprises 1-50% of a layer of a heat-sealable impact resistant polystyrene, up to 95% of a support layer, and 1-99% of a high melting plastic layer, wherein the sum of the thicknesses or weights of all layers is 100%.
European Patent 0 437 745 describes a sealable thermoplastic molding compound comprising at least four components: an impact resistant polystyrene resin, a block copolymer, a lubricant, and at least one homo- or copolymer of an aliphatic olefin. The sealable molding compound is applied to conventional support films, preferably comprised of polystyrene. The films are useful for sealing polystyrene or polyolefin (such as polyethylene or polypropylene) containers.
Unfortunately, multilayer films are expensive, difficult to dispose of properly and cannot be recycled. A heat-sealable film which is suitable for gastight sealing of plastic containers, particularly polystyrene containers, in a homogeneous layer and without additional surface treatment, is described in German Patent Applications P 41 42 691.6 and P 41 42 692.4. These films are directly sealable to polystyrene with the use of ordinary apparatus. These heat-sealable plastic films are based on polystyrene-compatible methacrylates comprising a molding compound with a two-phase structure. The grafting branches, and the ungrafted parts, of the impact resistant phase, are compatible with polystyrene. German Patent Applications P 41 42 691.6 and P 41 42 692.4 also relate to multilayer composite films wherein the above-mentioned molding compounds are processed to form support films and in a second step are coated with polystyrene-compatible molding compounds which have a two-phase structure.
However, in the case of multilayer composite films comprised of a support film produced from a polystyrene-compatible molding compound with a two-phase structure and a sealing layer comprised of a polystyrene-compatible readily flowable molding compound, a large difference in viscosity between the molding compound of the support layer and the molding compound of the sealing layer exists which creates problems in processing. Further, these (meth)acrylate films do not always have adequate tear strength under mechanical load.